Since an LOA is contingent on obtaining ANY nom and you have one and are 3 Q'ed, you should receive an appointment, not an LOA.
My gut feel is that the BGO system is showing an LOA vs. an appointment b/c USNA wants to give MOCs the first opportunity to congratulate you. To try to explain . . . you are considered sufficiently qualified to receive an LOA. In your case, you have already met the other conditions to receive an appointment so, instead of sending an LOA, they will send an appointment. However, b/c the Board decision was an LOA and b/c USNA wants the MOC to notify your first, the BGO system won't actually show the appointment until about 2 weeks after the certificate is sent -- and about a week after you receive it. This is why I tell candidates BGOs are usually the last to know.
[As an aside, b/c you received a Pres (vs. MOC) nom and b/c the Reps are all in hot campaign mode at the moment, don't be surprised if you don't receive a call or other notification from your MOC.]
I suggest you wait until you get whatever you get in the mail (LOA or appointment certificate). If it's an LOA, I recommend contacting your Regional Director and asking what else is necessary to receive an appointment -- b/c if you're 3 Q'ed, I can't think of anything.
It may simply be that it takes another week or so to "connect" the LOA with the other qualified items, but that would surprise me.